In relay assisted communication, relay nodes are deployed to forward information between a transmitter and a receiver. Different use cases can be identified for relay-assisted communication. The most popular are coverage extension, where a relay extends the coverage of a given base station, and traffic offload, and the base station can offload a part of its traffic to a relay for the relay to buffer and forward packets to a set of target users.
Co-channel interference is a critical problem regarding relay links using the same radio resources. 3GPP standardized Type I and II relays solve this problem using orthogonal resources for transmissions between the base station and the relays. However, this leads to a multiplexing loss that reduces the total network capacity.
In order to reduce the interference generated by in-band relay communication, different approaches are described in the literature. Usually, relay selection is combined with power control, sub-band allocation (see, for instance Jeon et al. “A survey on interference networks: Interference alignment and neutralization.” Entropy, vol. 14, no. 10, pp. 1842-1863, 2012) or transmit antenna selection (see, for instance Yang, Nan, et al. “Transmit antenna selection for security enhancement in MIMO wiretap channels.” IEEE Transactions on Communications, vol. 61, no. 1. pp. 144-154, 2013) in order to meet given interference constraints.
This selection can be formulated as an optimization problem to maximize a utility function, for example the average sum rate. In Lee et al. “Low-complexity interference-aware single relay selection in multi-source multi-destination cooperative networks.” 6th International Conference on Signal Processing and Communication Systems (ICSPCS), IEEE, 2012 the relay selection decision is formulated as an assignment problem, modeled as a weighted bipartite graph, and solved by the Hungarian algorithm. The algorithm input can be a channel state information (CSI) as for instance described in Cao, Jinlong, et al. “Interference-aware multi-user relay selection scheme in cooperative relay networks.” Globecom Workshops (GC Wkshps), IEEE, 2013. In Miao et al. “A Cross-Layer Relay Selection Algorithm for D2D Communication System.” International Conference on Wireless Communication and Sensor Network (WCSN), IEEE, 2014, an additional input, queue state information (QSI), is required.
However, the aforementioned approaches share two key disadvantages. Firstly, they require instantaneous channel knowledge, typically CSI, which leads to signaling costs and problems linked to outdated channel knowledge, and secondly, the approaches are characterized by high computational complexity mainly due to the necessity of solving large optimization problems in real time.
It is difficult to decrease the interference drawback without orthogonalizing the transmission and to efficiently select a relay, in such a way that the total interference level does not impact the network capacity in a destructive way.